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The US state is criss-crossed by thousands of miles of fibre-optics, which carry data signals to homes and businesses. But it's also in a high-risk earthquake zone, perched atop the San Andreas fault.
In a new study, researchers at California Institute of Technology were able to use a section of cable to measure the details of a magnitude-six earthquake, including pinpointing the time and location of four individual 'stuck' areas of the fault—known as asperities—that led to the rupture.
The work was conducted by geophysics professor Zhongwen Zhan and his team, who have been trying to repurpose fibre-optic cables as seismographs for many years, in a method called distributed acoustic sensing. The new study, which used only 100km of cable to analyse a 2021 earthquake, suggests better access to more cables would improve understanding of earthquake physics, and help provide better early-warning systems.
"If we can get broader coverage to measure seismic activity, we can revolutionise how we study earthquakes and provide more advance warning," says Zhan. "Though we cannot predict earthquakes, distributed acoustic sensing will lead to a better understanding of the details underlying how the earth ruptures."
Southern California has about 500 seismometers across a 56,500 square mile area, with each one costing up to $50,000. Using fibre-optic cables instead would be the equivalent of blanketing the state with millions of seismometers.
The technique works like this: laser emitters are positioned at one end of the cable, shooting beams of light through the long, thin glass strands that make up the core of the fibre optics. Tiny imperfections in the glass reflect a small amount of light back to the source, where it is recorded, meaning that each imperfection acts as a trackable waypoint along the cable.
These cables are generally buried just below the ground, so, when a seismic wave hits during an earthquake, the cables wiggle slightly, changing the amount of time it takes for a light beam to bounce back from each imperfection in the cable. Each tiny defect therefore acts as an individual seismometer.
"Using fibre-optic cable as a series of seismometers reveals aspects of earthquake physics that have long been hypothesised but difficult to image," says Zhan. "As an analogy, imagine your everyday backyard telescope. You can see Jupiter, but you probably can't see its moons or any details. With a really powerful telescope, you can see the fine details of the planet and moon surfaces. Our technology is like a powerful telescope for earthquakes."
The research was published in the journal
Nature.
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